The present invention relates to hindered-settling hydraulic classifiers and more particularly to a method and device for controlling the density of the teeter bed zone in a hindered-settling hydraulic classifier.
In hindered-settling hydraulic classifiers of the type generally described in United Kingdom Pat. No. 1,303,067 to Sidebottom, a zone or bed of particles is maintained in a densely composed teetering condition by an upward flow of water. This zone is referred to as a teeter bed zone and functions as the primary medium for particle size separation with finer particles remaining above the teeter bed zone for overflow removal and coarser particles settling downwardly through the teeter bed zone for underflow discharge removal. In wet classifying applications, the teeter bed zone is composed of particles having a structual size close to and somewhat coarser than the desired size of separation or classification of feed slurry particles, i.e., the desired cut point of classification. Maintenance of the proper density of the teeter bed zone is necessary for control of the operation of the classifier and to minimize the amount of overflow of coarse material with fine material and the amount of downward settling of fine material with coarse material.
Control of the density of the teeter bed zone is desirable for achieving a sharp product split and efficient classification of material as well as for minimizing the amount of fine material settling through the teeter bed zone with the coarse material. Prior devices such as the type disclosed in United Kingdom Pat. No. 1,303,067 generally utilized a single sensing unit for sensing density within the classifying chamber. Similar hindered-settling devices operated for specific gravity separation or concentration (rather than size classification) also utilize a single sensing unit for controlling the teeter bed zone, e.g., U.S. Pat. No. 4,282,088 to Ennis. Consequently, the control and maintenance of the teeter bed zone was based upon the measurement of a single sensing device which functioned to control the underflow discharge outlet. The accuracy of the sensing unit was however restricted by undesirable factors such as feed slurry density, feed fluctuations, etc., which adversely affected regulation of the underflow discharge outlet and thus the control of the density of the teeter bed zone.
Accordingly, it is a principal object of the present invention to provide a teeter bed density control device and method which provides accurate measurement and control of teeter bed zone density.
Another object of the invention is to provide a teeter bed density control device and method which accomplishes a more efficient classification of materials with sharper product splits and reduces the amount of fine material reporting with coarse material at higher feed loading rates.
A further object of the invention is to provide a teeter bed density control device and method which affords flexibility through facile variation of the composition of the products, close control over the size structure of the coarse underflow product, accurate control of the cut point of the separation, and convenient preselection of teeter bed density.
A still further object of the invention is to provide a teeter bed density control and method which compensates for variations in feed tonage and particle size composition while still maintaining the desired product split characteristics.
The foregoing and related objects are accomplished in a hindered-settling hydraulic classifier utilizing the teeter bed density control device of the present invention which includes a first sensor for measuring the average slurry density from the upper boundary of the teeter bed zone to the top of the overflow zone, a second sensor for measuring the average slurry density from the lower boundary of the teeter bed zone to the top of the overflow zone, and a controller connected to the sensors for generating a compensating control signal based upon a preselected density standard and the average slurry density measurements. An actuable control valve for regulating the flow of discharge through the underflow discharge outlet is operationally connected to the controller to regulate discharge responsive to the compensating control signal.
In the method of the present invention for controlling the density of the teeter bed zone, the desired value of average teeter bed zone density is selected and the average slurry density is measured from the upper boundary of the teeter bed zone to top of the overflow zone and from the lower boundary of the teeter bed zone to the top of the overflow zone. The underflow discharge is regulated in proportion to the measured average slurry densities to adjust the average density of the teeter bed zone to the selected value. Measuring the average slurry densities includes the determination of the average teeter bed zone density with the regulation of the underflow discharge being based upon the average value and the selected value of the teeter bed zone density.